Thanks for the credentials, it seems the issue is intermittent. It appears sometimes and sometimes it works fine, which confirms this is not a bug/issue in the theme.
We have been able to narrow down the bug to the following :
a. The page builder makes a ajax call via admin-ajax.php to save the content of the page .
b. Sometimes the admin-ajax.php returns a 404, refer screenshot.
admin-ajax.php is a core file or WordPress.
refer : http://i.imgur.com/XS1kPQo.png
Please consult your web host in the issue.
Also try checking the following topic in WordPress.org : https://wordpress.org/support/topic/admin-ajaxphp-404-error-after-update-to-402